Field
The disclosed concept pertains generally to voltage regulators used to stabilize the voltage being supplied in a power distribution system, and, more particularly, to a voltage regulator and method of controlling same that is able to accommodate the blending of a forward cogeneration mode of operation and a reverse power flow mode of operation due to system reconfiguration.
Background Information
The power distribution industry uses systems comprising a network of power lines to distribute electrical power to homes and businesses. In these power distribution systems, it is important that the electrical power be supplied continuously and at a stable voltage level. If the supply of power is not stable and continuous, then consumers will experience problems such as flickering lights and the malfunction of electrical equipment such as computers.
One problem that power companies face in trying to consistently supply stable and continuous power results from the fact that the electrical loads of their consumers are constantly changing. In particular, increased electrical loads on a power distribution system will have the tendency to reduce the voltage level of the supplied power. Likewise, decreased electrical loads on a power distribution system will tend to increase the voltage level of the supplied power.
To compensate for the changing voltage levels caused by changes in electrical load, power companies employ voltage regulating equipment to raise the voltage level in response to an increase in load and to decrease the voltage level in response to a decrease in load. A voltage regulating device is a power quality device that provides a stable output voltage despite fluctuations in an input voltage. A common type of voltage regulating device is what is known as a transformer equipped with a load tap changer (LTC), typically located at distribution substations. Step voltage regulators, an autotransformer, may also be used in distribution substations and also on single power line feeders. For example, if an input voltage fluctuates between 110 VAC and 130 VAC, the voltage regulating device maintains the output voltage at a constant 120 VAC. The voltage regulating device operates by comparing the actual output voltage (which is either measured directly or calculated) to a fixed reference voltage set point (a user-defined setting). The reference voltage set point is typically stored within a voltage regulator control unit, which controls operation of the voltage regulator. The voltage regulator control unit determines the difference between the actual output voltage and the reference voltage set point and uses this difference to control a regulating element. The regulating element is typically a tap changer that establishes and varies as needed the winding ratio between a primary and a secondary transformer winding, or a series and shunt winding (in the case of a step voltage regulator). A motor controls a position of the tap changer, and operating the tap changer changes the winding ratio and thus output voltage. The voltage regulator control unit controls the position of the tap changer to reduce the difference between the regulator output voltage and the set point to a value within a user-defined bandwidth, typically between about 1 and 6 volts.
Evolving applications on utility distribution grids have increased the complexity of the required functionality of a voltage regulator control where two distinct modes of operation, namely cogeneration and reverse power flow due to system reconfiguration, can be required out of the same connected and configured devices. This is illustrated with reference to FIGS. 1 and 2. More specifically, FIG. 1 shows an electrical distribution system 1 in a normal bus configuration having a first feeder circuit 2A and a second feeder circuit 2B. As seen in FIG. 1, feeder circuit 2A is fed from a voltage supply (e.g., a substation) 3A and a normally closed switch 4A in order to feed a number of loads 5A and a critical load center 6A. Also connected to feeder circuit 2A is generator 7, which may be a distributed source such as a wind turbine or a PV (photo voltaic) module. Feeder circuit 2B is fed through a voltage supply 3B and a normally closed switch 4B in order to feed a number of loads 5B and a critical load center 6B. Feeder circuit 2A and feeder circuit 2B are separated from one another by a normally open switch 8. Voltage supplies 3A and 3B may be fed from the same or different electrical substations.
Voltage regulators 9A and 9B are located on respective feeders 2A and 2B to support voltage regulation downstream on the feeder. FIG. 2 shows electrical distribution system 1 in a condition wherein it has been reconfigured into a back fed bus configuration. In this configuration, normally closed switch 4A is opened in order to remove voltage supply 3A from service, and normally open switch 8 is closed. As a result, feeder circuit 2B and feeder circuit 2A will both be fed by voltage supply 3B.
In the system configuration as shown in FIG. 2, a normally prescribed method of Cogeneration operation can and will cause the Voltage Regulator to actually drive the controlled voltage in the opposite direction of that which is needed to regulate voltage to loads downstream of the Voltage Regulator in this reverse power flow scenario.
No current method of operating and/or controlling a voltage regulator exists that is capable of accommodating the blending of a cogeneration mode of operation and a reverse power flow due to system reconfiguration mode of operation. There is thus a need for a voltage regulator and method of controlling same that is able to accommodate the blending of these two distinct modes of operation.